Advances in Electrolytic Manganese Residue: Harmless Treatment and Comprehensive Utilization

Abstract

Electrolytic manganese residue (EMR) is a byproduct of electrolytic manganese production, rich in soluble pollutants such as manganese and ammonia nitrogen. Traditional stockpiling methods result in contaminant leaching and water pollution, threatening ecosystems. Meanwhile, EMR has significant resource-recovery potential. This paper systematically reviews the harmless process and resource technology of EMR, efficiency bottlenecks, and the current status of industrial applications. The mechanisms of chemical leaching, precipitation, solidification, roasting, electrochemistry, and microorganisms were analyzed. Among these, electrochemical purification stands out for its efficiency and environmental benefits, positioning it as a promising option for broad industrial use. The mechanisms of chemical leaching, precipitation, solidification, roasting, electrochemistry, and microorganisms were analyzed, revealing the complementarity between building materials and chemical materials (microcrystalline glass) in scale and high-value-added production. But the lack of impurity separation accuracy and market standards restricts its promotion. Finally, it proposes future directions for EMR resource utilization based on practical and economic considerations.

1. Introduction

Manganese is a vital material in the national economy and one of the key strategic resources. As the world’s largest producer, China accounts for 94% of global manganese metal output. Electrolytic manganese residue (EMR) is a solid waste byproduct generated during the solid–liquid separation process in the electrolytic production of metallic manganese [1]. For every ton of manganese metal produced, approximately 7 to 9 tons of manganese residue are generated [2]. The accumulation of EMR has exceeded 50 million tons and continues to grow by nearly 10 million tons annually [3,4,5,6]. The predominant disposal method involves extensive landfilling, which leads to the leaching of NH₃-N and heavy metals, such as Mn²⁺, Zn²⁺, Cu²⁺, Pb²⁺, Ni²⁺, and Co²⁺ into the soil, thereby contaminating nearby rivers and groundwater. This pollution poses significant risks to local communities and ecosystems. Removing the impurities and curing EMR can enhance its application in traditional industries, such as construction materials. However, these methods are classified as low-value utilization and will cause the waste of resources. Studying the utilization of mineral elements in EMR is critical for sustainable development, including reducing accumulation and mitigating environmental risks [7].

The efficient utilization of manganese residue encounters two primary challenges. First, its complex impurity composition raises the cost of extracting valuable components. Second, the low value-added nature of its products significantly limits marketability. Enhancing the comprehensive recovery and utilization of manganese ore resources, particularly EMR and manganese tailings, is essential. Achieving the optimal allocation and sustainable development of manganese ore resources while maintaining an ecological balance remains a critical challenge for mineral processing researchers. This article critically reviews EMR harmlessness approaches, proposes strategies to mitigate EMR accumulation, and discusses the challenges and prospects of resource utilization. The aim is to provide insights for the large-scale disposal of EMR.

2. EMR Properties

2.1. Production

EMR is the waste residue produced from various processes in electrolytic manganese industrial production, including sulfuric acid leaching residues, neutralization residues, electrolytic deposition residues (anode slime), and pressure filter residues [8], and it is characterized by significant contents of Mn, Ca, K, Mg, Na, Si, and trace amounts of heavy metals. The main constituent of the anode slime generated in this process is MnO. After drying, this material can be employed in various industrial applications. While most manganese oxides can be effectively recycled, the recycling of manganese residues in other processes remains limited [9].

In China’s electrolytic manganese industrial production, most raw materials consist of low-grade manganese ore, primarily MnCO₃, typically available in large quantities [10]. Due to the low grade of manganese ore in China, metallic manganese is primarily produced through hydrometallurgical processes, as illustrated in Figure 1. The manganese ore is crushed and ball-milled into powder and enters the magnetic separation system to separate the concentrate. The powder is reacted with sulfuric acid to convert manganese carbonate into manganese sulfate. The acid leaching process produces acid leaching residue containing ferroaluminosilicate. Subsequently, neutralization and sulfidation are carried out to remove impurities. In the neutralization stage, oxidants and ammonia water are added to precipitate Fe³⁺ and Al³⁺. In the sulfidation stage, Na₂S is added to precipitate Pb²⁺ and Cd²⁺. Therefore, the waste manganese residue from these two stages contains corresponding precipitated impurities. The electrolyte is added to the manganese sulfate after impurities are removed to promote cathode manganese precipitation, produce anode mud containing PbO₂ and MnO₂, and finally obtain the electrolytic manganese product through filter pressing, passivation, and drying.

2.2. Physicochemical Properties

EMR is a dark, powdery solid waste material. As illustrated in

Table 1. Chemical composition of EMR (wt%).

Area	SiO2	SO3	CaO	Al2O3	MnO	Fe2O3
Hunan [16]	20.01	19.26	9.28	9.53	4.64	5.17
Chongqing [17]	32.32	30.77	14.27	7.63	3.00	6.32
Guangxi [18]	17.48	32.04	14.19	1.93	6.89	7.63
Chongqing Xiushan [19]	38.55	27.32	9.34	7.78	3.11	6.58

[11] and Figure 2 [12], its primary elemental composition includes O, Si, S, Ca, Al, Mn, and Fe, among others. The main minerals in EMR are gypsum, quartz, mica, and albite [13]. Its main chemical components are CaO, Al₂O₃, SiO₂, and SO₃ [14]. EMR exhibits a high water-retention capacity, with a moisture content of 20–30 wt% [12]. Prolonged exposure to rainwater can lead to the formation of a slurry, further increasing its porosity. The pH value of the leachate varies from 5.9 to 6.6, indicating a slightly acidic nature [12,15].

2.3. Leaching Risk

EMR is primarily treated using stacking and landfilling, with the total stockpile in the country exceeding 50 million tons [20]. Figure 3 shows the three-dimensional spread of EMR pollution [21,22]. Stockpiled EMR not only occupies land resources but also releases harmful substances into the soil and water system after long-term weathering and leaching, thereby polluting the surrounding ecosystem 20–22 [23,24,25]. In addition, EMR exhibits acidic properties. Large amounts of stockpiled EMR infiltrate into the surrounding soil through rainwater runoff, causing soil acidification and structural degradation [26,27].

Table 2. The leaching results of EMR (mg/L).

Serial Number	Mn2+	NH3-N	Pb2+	Cd2+	Cu2+	Zn2+
1	1300	651	/	0.036	0.051	1.14
2	1220	140	0.013	0.073	0.464	1.7
3	1414	502	0.745	0.034	0.091	0.453
4	912	1 030	0.24	0.05	/	0.76
GB 8978-1996	5	25	1.0	0.1	0.5	5.0

presents the toxicity leaching results of EMR as reported in four studies [1]. It is evident that the concentrations of Mn²⁺ and NH₃-N in the leaching solutions of EMR significantly exceed the limits established in the IWDS (GB 8978-1996 [28]). Therefore, during the process of the harmless treatment and resource utilization of EMR, Mn²⁺ and NH₃-N are the critical indicators and focal points of research. The concentrations of its primary pollutants (such as Mn²⁺ and NH₃-N) have surpassed safe thresholds, while the levels of As, Hg, and Se are also elevated at present [29].

3. EMR Harmless Process

3.1. Chemical Method

The chemical method uses chemical reactions to change the chemical properties of substances. During this process, EMR undergoes chemical changes such as decomposition, oxidation, and reduction. Impurities in EMR may either precipitate and separate or leach out for recycling.

3.1.1. Chemical Precipitation

Precipitation is a method of separating the target solute from the solution by adding specific chemical reagents to the solution so that the target solute reacts chemically with the reagent to form an insoluble precipitate (such as soluble Mn²⁺ and NH₄⁺-N), which can separate the target solute from the solution. Shu et al. [30] used a combination of MgO and phosphate reagents to leach Mn²⁺ and another major pollutant component NH₄⁺-N, achieving efficiencies of 91.58% and 99.98%, respectively. After stabilization, Mn²⁺ exists as MnHPO₄·2H₂O, Mn(H₂PO₄)₂·2H₂O, Mn₃(PO₄)₂·3H₂O, Mn(OH)₂, and MnOOH, while NH₄⁺-N is present as NH₄MgPO₄·6H₂O and NH₄MnPO₄·6H₂O. The precipitation method is highly targeted and can recover specific impurities. The process is relatively mature and suitable for large-scale treatment, but it will produce secondary pollutants (precipitates) that require additional disposal, increasing treatment costs. It also has strict pH and metering control requirements and complex operations.

3.1.2. Chemical Leaching

Chemical leaching can be roughly divided into acid leaching and alkaline leaching. Acid leaching is a process that utilizes acidic solutions (such as H₂SO₄, HCl, and HNO₃) or other chemical reagents (such as oxalic acid and ammonium persulfate) to dissolve and extract specific impurities or valuable metals in EMR [31]. When the concentration of H₂C₂O₄ exceeds 0.31 mol/L, the manganese leaching rate in EMR levels off at 83.6%. Meanwhile, it can be seen that increasing the temperature from 25 to 90 °C increases the recovery of iron from 10% to 35.6%. H₂C₂O₄ reacts with high-valent manganese oxides in the residue to form Mn²⁺ [32]. Under the influence of Fe(OH)₃, an iron complex is formed, initiating a reduction reaction that converts trivalent iron to divalent iron. The iron complex is transformed, and the divalent iron complex dissociates into Fe²⁺ under acidic conditions [33], thereby enhancing the manganese leaching rate. After reacting at 85 °C for 120 min, the recovery rates of Mn and Fe were nearly 100% and 79.3%, respectively [34].

Alkaline leaching is less common and yields lower extraction rates compared to acid leaching. It is typically used for elements like Fe, Al, and Si. Zou et al. [35] utilized a 2 mol/L NaOH solution to leach impurities from EMR at a temperature of 130 °C, with a stirring speed of 300 r/min for 5 h. The Si leaching rate achieved was 82.04%. The reaction between the EMR and NaOH solution is a non-catalytic, multi-phase liquid–solid reaction. The chemical reaction is as follows:

$$2NaO{H}_{\left(aq\right)}+nSi{O}_{2\left(s\right)}=N{a}_2{O\cdot nSi{O}_2}_{\left(aq\right)}+H_{2}O$$

(1)

$$M{n}_{(aq)}^{2+}+2O{H}_{(aq)}^{-}=Mn{(OH)}_{2(S)}\downarrow$$

(2)

In EMR with high soluble impurities, water pretreatment followed by chemical leaching can help improve the leaching efficiency. In summary, both acid and alkali leaching can effectively recover most Mn and NH₄⁺-N from EMR. From the perspectives of the treatment cost and environmental protection, water leaching presents more advantages, as acid leaching can lead to secondary pollution. In terms of comprehensive utilization, the valuable elements extracted from acid-leached EMR, such as NH4⁺-N, calcium, iron, and manganese, can be utilized as nutrients for plants. Li et al. [36] utilized different amounts of H₂SO₄ to separate the organic phase, as shown in

Table 3. Stripping results using H₂SO₄ [36].

H2SO4 Concentration (mol/L)	Stripping Solution (mol/L)			Stripping Ratio (S,%)
	Mn2+	Mg2+	NH4+	Mn2+	Mg2+	NH4+
0.00	0.0023	0.0005	0.0176	5.2	40.3	29.4
0.01	0.0046	0.0008	0.0188	10.8	60.8	31.2
0.05	0.0089	0.0009	0.0208	20.5	70.1	34.7
0.10	0.0161	0.0010	0.0232	37.2	73.6	38.8
0.50	0.0349	0.0010	0.0436	80.6	77.4	72.6
1.00	0.0413	0.0010	0.0582	95.3	78.3	97.0

(O/A = 1:2, temperature = 25 °C, and contact time = 10 min). Trace and valuable elements do not require separate separation processes. The acid-leaching solution can be directly used as fertilizer after adjusting its pH. Furthermore, the residue remaining after acid leaching primarily consists of plagioclase, which can serve as a raw material for ceramics, refractory materials, and glass [37]. From the standpoint of impurity removal, the Si and Se in EMR are mainly eliminated through alkali leaching. However, the potential for secondary environmental pollution and high energy consumption imposes limitations on the EMR processing due to economic pressures.

3.2. Physical Chemistry Method

The physicochemical method is a method that uses the comprehensive process of physical and chemical reactions to change the physical and chemical properties of substances. It can be an independent treatment system or a subsequent treatment facility after biological treatment, which can deal with complex and diversified EMR impurities through comprehensive physical and chemical treatment.

3.2.1. Curing

Inert solidification materials, such as cement, are utilized to encapsulate harmful components in EMR and reduce their mobility [10]. This principle forms the foundation of EMR solidification treatment technology [24]. Typically, cement with a minimum content of 45% is used as the primary solidification material [38]. Shu et al. [39] proposed using an alkaline material produced through cement sintering to solidify Mn and NH₄⁺-N, as shown in Figure 4 [39]. Luo et al. [16] mixed quicklime and EMR in a 1:9 mass ratio, pre-stirred the mixture for 40 min, and allowed it to stand for 3 h. The solidification rate of soluble Mn in EMR was 99.8%, while the solidification rate for NH₄⁺-N was 96.73%. The concentrations of Mn and NH₄⁺-N in the solidification leachate were 2.60 mg/L and 21.23 mg/L, respectively, as shown in Figure 5a,b [16]. Zhou et al. [40] added quicklime and NaOH to solidify Mn in EMR and remove NH₄⁺-N. The results indicated that adding CaO was more effective than NaOH in coagulating Mn and removing NH₄⁺-N. The manganese solidification rate and NH₄⁺-N removal rate reached 99.98% and 99.21%, respectively. Pan et al. [41] added Na₂SiO₄ and utilized saturated clarified lime water to adjust the pH and solidify the EMR. The findings demonstrated that Na₂SiO₄ significantly influenced the solidification rate of soluble Mn in EMR. Under controlled conditions, the solidification rate of soluble Mn in EMR was 96.8%, and the residual Mn concentration in the filtrate was 0.99 mg/L, as shown in Figure 5c [41].

Solidification and stabilization technology can offer a short-term, harmless treatment for EMR. The advantages of this method are a simple process, high industrial maturity, low processing cost, and low risk of leaching of harmful impurities, but its disadvantages are low resource utilization, being a low-value process, and resulting in the waste of resources.

3.2.2. High-Temperature Method

EMR directly decomposes harmful components such as NH₄⁺ and SO₄²⁻ through high-temperature calcination or sintering, making the residue harmless, or it changes the physical and chemical properties of EMR for stabilization and separation; for example, Fe₂O₃ and Mn₂O₃ are reduced and roasted into Fe₃O₄ and Mn₃O₄, and then, the iron and manganese oxides are separated via magnetic separation. Zhang et al. [42] conducted direct high-temperature calcination and reduction roasting using carbon powder on EMR. At temperatures exceeding 600 °C, EMR decomposes sulfate salts, and NH₄⁺ decomposes, releasing NH₃. The reaction Equations (3)–(6) for some chemical components of EMR at high temperatures are as follows:

$$2(N{H}_4{)}_{2}S{O}_4\triangleq 4N{H}_3\uparrow +2S{O}_2\uparrow +2H_{2}O+O_2\uparrow$$

(3)

$$MnS{O}_4\triangleq Mn{O}_2+S{O}_2\uparrow$$

(4)

$$2(N{H}_4{)}_{2}S{O}_4+C\triangleq 4N{H}_3\uparrow +2S{O}_2\uparrow +2H_{2}O+C{O}_2\uparrow$$

(5)

$$2MnS{O}_4+C\triangleq 2Mn{O}_2+2S{O}_2\uparrow +C{O}_2\uparrow$$

(6)

Wang et al. [43] found that pyrometallurgical treatment can minimize pollutants in EMR, ensuring that the heavy metal content complies with national standards. Yuan et al. [44] calcined the EMR at 650 °C for 40 min in an atmosphere of air and CO (the CO concentration of 35%), followed by magnetic separation. In this process, the main useful elements in the magnetite concentrate were the iron element, accounting for 75.47%, while the main useful elements in the magnetite tailings were the manganese element, accounting for 88.75%. The principles of separation and recovery are illustrated in Figure 6 [44]. Sun et al. [45] utilized coke as a reducing agent to recover sulfur from EMR through high-temperature roasting. The S recovery rate from EMR reached 99.7%.

Roasting EMR not only removes harmful components but also activates beneficial ones, thereby enhancing resource utilization. Incorporating an appropriate amount of reducing carbon powder during the roasting process lowers the decomposition temperature of common sulfates found in the manganese residue, which in turn reduces processing costs. However, the high-temperature method consumes a lot of energy and requires the support of high-temperature equipment. It is important to note that ammonium salts and other ammonium-containing substances contained in the manganese residue may decompose and release NH₃ at elevated temperatures. Therefore, gas treatment processes must be properly aligned to prevent secondary pollution.

3.3. Electrochemical Method

In recent years, electrochemical extraction has been extensively utilized for the extraction of vanadium from converter manganese residue, copper extraction, remediation of heavy metal-contaminated soil, and recovery of manganese from manganese-containing resources [46,47]. Electromigration and electroosmosis are the main mechanisms for the removal of manganese and ammonia nitrogen in EMR. By changing the existence form of impurities through anodic oxidation or cathodic reduction, the ionic strength and porosity are enhanced, which boosts electromigration. Electroosmosis is increased by altering the zeta potential. Shu et al. [47] added EMR into an acid-leaching solution, subsequently adding sodium dodecyl benzene sulfonate, citric acid, EDTA, and other reagents in specific molar ratios. After applying a pulsed electric field for 84 h, the maximum removal rates of Mn²⁺ and NH₄⁺ in EMR reached 94.74% and 88.20%, respectively (Figure 7a [47]). Tian et al. [13] utilized a leaching solution composed of 9.15 wt.% H₂SO₄ and 3.33 wt.% H₂O₂. Under acidic conditions and in the presence of Fe²⁺, H₂O₂ catalyzes the generation of hydroxyl radicals (-OH), which possess strong oxidizing capabilities and can significantly enhance the leaching rates of metal ions and NH₄⁺-N. When the current density was set at 35 mA/cm², the leaching rates of Mn and NH₄⁺-N reached 88.07% and 91.50%, respectively, after 120 min of reaction at 40 °C (Figure 7c [13]). Shu et al. [48] used 13 wt.% H₂SO₄ as the leachate and applied an external electric field to enhance the process. After a reaction time of 120 min at 20 °C, the leaching rates of Mn and NH₄⁺-N were 89.4% and 65.9%, respectively (Figure 7d [49]). Following the addition of 100 mg/L of TTC and applying an electric field, the leaching rates of Mn and NH₄⁺-N increased to 97.1% and 98.4%.

Although electric field enhancers can improve the leaching rate, this process requires substantial energy and produces a large volume of electrolyte [50]. Additionally, the anode generates chlorine gas, which can result in secondary contamination [51]. In conclusion, using traditional electrochemical methods to extract manganese from EMR encounters challenges such as high energy consumption and low processing efficiency due to various limiting factors.

3.4. Biological Method

Bioleaching is a process used to extract valuable elements from ores or solid waste. As a recycling technique, it offers significant potential for environmental protection. Duan et al. [52] utilized sulfur-oxidizing and iron-oxidizing bacteria as culture media, using sulfur and pyrite as sources of S and Fe, to bioleach EMR. In this process, EMR is subjected to a sulfur–iron-oxidizing bacterial leaching system for 9 h, resulting in a Mn leaching rate of 99.7%. Lv et al. [53] discovered that Bacillus myxogenes and Bacillus circulans can activate Si and immobilize heavy metal components in EMR. The results indicate that the direct contact efficiency between the bacteria and EMR is high, with an effective Si leaching concentration of 163.27 mg/L (Figure 8a [53]). Most researchers have utilized specific bacterial species for the bioleaching of EMR [52], while others have isolated bacteria from the residue for the leaching process. Lan et al. [54] cultured an EMR suspension for 3–5 days, subsequently purifying and isolating the natural bacterial strain (Y1). They utilized bacteria Y1 along with waste molasses as substrates, successfully recovering valuable elements, such as SO²⁻, Mg, Mn, Fe, and NH₄⁺, from the residue after 8 days of bioleaching. Through this method, the researchers achieved the recovery of higher-purity (NH₄)₂Mn(SO₄)₂·6(H₂O), (NH₄)₂Fe(SO₄)₂·6(H₂O), and (NH₄)₂Mg(SO₄)₂·6(H₂O) from the leachate (Figure 8b [54]).

Bioleaching, a traditional hydrometallurgical process, effectively recovers low-grade elements from waste materials. It is highly selective and suitable for the extraction of specific impurities. Compared to traditional physical and chemical leaching methods, bioleaching is simpler, more cost-effective, and more environmentally friendly [55]. However, it necessitates a high liquid-to-solid ratio and the cultivation of bacteria under challenging environmental conditions [56].

3.5. Comprehensive Method

As shown in

Table 4. EMR harmless processes.

Method	Process	Advantages	Disadvantages
Chemistry	Precipitation	strong targeting; mature technology; large-scale application	produces secondary pollution; strict pH and metering requirements; complex operation
	Leaching	extractable valuable elements; high removal efficiency; EMR can be used as building materials after leaching	produces secondary pollution; acidic wastewater treatment is difficult
Physical chemistry	Curing	simple and mature process; low cost; low leaching risk	causes waste of resources
	High-temperature treatment	strong targeting; high removal efficiency; after roasting, EMR can be used as building materials	high energy consumption; harmful gases from roasting need to be recovered
Electrochemistry	Electromigration/ Electroosmosis	environmentally friendly; relatively low pollution; strong targeting	high energy consumption; specific equipment required
Biology	Microorganism	economical and environmentally friendly; strong targeting; no chemical reagents required	no secondary pollution; bacterial culture complex; environmentally sensitive

, the harmless treatment processes of EMR each have their advantages and disadvantages. Currently, no single process technology can effectively remove all pollutants. Therefore, many researchers have carried out comprehensive harmless treatment processes for EMR to deal with the complex and diverse impurity system. Research has demonstrated that the combination of water and acid leaching significantly enhances manganese recovery. When Lan et al. [57] conducted the ball milling leaching of manganese in EMR, using optimal process parameters, the leaching rate of manganese exceeded 95.56%. During the water leaching process, the addition of dihydrate oxalic acid to the ball mill resulted in a manganese leaching rate of over 98%. The concentration of manganese in EMR after ball milling was below the concentration limit set by the Comprehensive Wastewater Discharge Standard (GB 8978-1996), facilitating the conversion of EMR from Type II solid waste to Type I. The mechanical–chemical-strengthening mechanism for the selective recovery of manganese from electrolytic manganese residue is shown in Figure 9 [57].

Huang [58] first extracted a significant amount of metal ions from the leaching solution produced from EMR using Fusarium sp. Subsequently, manganese ions were purified and extracted from this solution using NaOH and HCl. The precipitated substance (Mn₂O₃) was then heated to produce MnCl₂, achieving a purity level ranging from over 76% to as high as 93%. This study addresses the limitations of traditional chemical methods for manganese recovery by employing a biological approach, which offers enhanced energy efficiency. Additionally, it mitigates the drawbacks of the microbial leaching process in manganese extraction by incorporating chemical methods, resulting in a relatively straightforward operation. The industrial process flow chart for the biological–chemical recovery of manganese is illustrated in Figure 10 [58].

4. Comprehensive Utilization of EMR

The building materials industry has long served as a primary avenue for the consumption of solid waste. Furthermore, the precious metals found in EMR can be repurposed for valuable applications, including element recovery, manganese fertilizer production, microcrystalline glass manufacturing, and wastewater treatment, as shown in Figure 11. This chapter concludes with a summary of the comprehensive utilization of EMR, as shown in

Table 5. Summary of the comprehensive utilization of EMR.

Type	Conclusion
Recovery of valuable elements	Manganese residue contains very little soluble manganese and ammonia nitrogen, and its recovery process is relatively complicated, with many influencing factors.
Manganese fertilizer	Research has proven that using manganese residue to produce manganese fertilizer is feasible, benefiting agricultural development and enhancing the resource utilization of EMR [59].
Glass–ceramic	The preparation method of EMR glass–ceramics is simple, energy-efficient, and environmentally friendly, making it a promising direction for EMR resource utilization.
Building materials	The preparation of cement concrete and wall materials from EMR could be an important turning point in solving the storage of EMR.
Road base material	EMR as a roadbed material reduces the natural clay consumption while effectively utilizing EMR.
Wastewater treatment	The application of EMR in wastewater pollutant adsorption provides a reference for developing high-value-added products.

.

4.1. Building Materials

EMR contains significant amounts of soluble sulfate, which can stimulate volcanic ash materials to produce energy-efficient, unburned brick, autoclaved bricks, cement additives, and concrete additives, as shown in

Table 6. Applications of EMR in the building materials industry.

Application	Related Products	Manganese Residue Dosage	Weaknesses
Cement additives	Cement mineralizer, cement mixture, residue cement, sulfoaluminate-like cement, high iron sulfoaluminate cement	3–5%	The ammonia removal and desulfurization processes are not yet mature, and the cost is relatively high.
Concrete additives	Composite admixtures, sulfate activators, sulfur concrete fillers	<10%	Low activity and lack of low-cost, efficient activation technology.
Wall material	Unburned bricks, autoclaved bricks, non-burned permeable bricks, and autoclaved aerated concrete	30–60%	Without ammonia removal treatment, these products will suffer severe frost in humid environments; deammoniation costs are high, and market potential is limited.

. The utilization of EMR in building materials is a key area of current research.

4.1.1. Cement and Concrete Additives

EMR is primarily utilized as an additive in cement, including retarders, mineralizers, light aggregates, and cementitious materials. Guan [60] investigated the main components of EMR and utilized it as a sustained-release agent in cement production. They discovered that the primary constituents in EMR, such as silica and gypsum, can effectively function as sustained-release agents in cement manufacturing. Wang et al. [61] examined the application of EMR as a cement additive and found that manganese residue calcined at 450–750 °C exhibits significant dehydrated gypsum and pozzolanic activity. During the cement preparation process, 15% of the calcined residue was incorporated, and the resulting products met the technical specifications for grade 42.5 ordinary Portland cement. The preparation process for EMR cement is illustrated in Figure 12a. Lei et al. [62] produced high-iron sulfoaluminate cement using CaCO₃ and Al₂O₃ derived from EMR as raw materials, adding 25% residue and calcining at 1200 °C for 60 min. This method achieved optimal cement performance, with a 3-day compressive strength of 49.8 MPa. Zhao et al. [63] utilized EMR and magnesium residue to prepare a sulfoaluminate cement clinker, with the mixing ratio of the two waste residues reaching 21% each. The resulting cement demonstrated excellent mechanical properties and an impermeability grade of P6. Liu et al. [64] found that high-temperature calcination at 300 °C enhances the retarding effect of EMR. After high-temperature calcination, EMR can fully replace natural gypsum in cement production, with its performance meeting national standards.

Under ambient temperature and humidity, static EMR reacts with lime to form hydrates that possess hydraulic cementitious properties. This reaction imparts pozzolanic activity to the EMR, thereby enhancing the performance of concrete. Additionally, the sulfates present in static EMR can activate certain low-activity mineral components [65]. Consequently, EMR can be utilized as a blending material in concrete and as an additive, such as a sulfate activator [66]. Chousidis et al. [67] successfully prepared C25/C3 concrete with 5% to 10% EMR contents. In addition to demonstrating good compressive strength, the concrete also exhibits resistance to chloride ion erosion. Giergiczny et al. [68] successfully developed a novel adhesive by mixing blast furnace residue, calcium hydroxide, water, and EMR in specific proportions, followed by the addition of distilled water and solidification at 25 °C and 95% humidity for 72 h. The resulting material is shown in Figure 12b. This process also stabilizes Mn²⁺ and NH₄⁺.

After proper treatment (pretreatment is required if there are serious pollutants), the performance of EMR can meet the requirements of cement and concrete production, and is even superior to natural gypsum in some aspects. The overall cost is lower than that of natural gypsum, and the use of EMR can also enjoy tax-reduction policies, further reducing corporate costs and encouraging resource utilization. But the limited availability of EMR admixtures and their inability to be widely applied on a large scale result in minimal environmental benefits. Therefore, the large-scale adoption of EMR in building materials continues to face significant challenges.

4.1.2. Wall Materials

EMR contains active ingredients such as silicon–aluminum compounds, which can undergo a volcanic ash reaction in an alkaline environment to generate hydration products with gelling properties, thereby improving the strength and performance of the brick. The preparation of non-sintered permeable bricks involves the use of materials such as EMR, blast furnace residue, cement, gravel, and various additives. Wang et al. [69] investigated the production of these non-sintered permeable bricks, which can contain up to 60% EMR and have been successfully utilized in pavements applications. They also examined the pore structure of permeable bricks containing 15% EMR. The structure is illustrated in Figure 13a [69]. Lan et al. [70] utilized mechanical activation to activate silicon in EMR, promote NH₄⁺ removal, and prepare unfired bricks with a compressive strength of 12 MPa, as shown in Figure 13b [49]. Jiang [71] utilized an EMR–fly ash–lime–cement system to create residue-free bricks and investigated the effects of the mortar-to-sand ratio and molding pressure on the mechanical properties of manganese-residue bricks. The results indicate that, with a mortar-to-sand ratio of 1:0.9, a water-to-solid ratio of 0.14, and a molding pressure of 25 MPa, the compressive strength of unfired bricks can reach 17 MPa after 28 days. Guo [72] examined the impact of varying amounts of fly ash and cement on the mechanical properties of the lime–manganese residue gelling system. The findings reveal that mixing 60% manganese residue, 10% lime, 20% fly ash, and 10% cement by mass results in unfired bricks with excellent mechanical properties. Wang [61] used EMR and a small amount of cement to produce steam-cured bricks with a compressive strength of 26 MPa by incorporating siliceous materials and quicklime. Qin et al. [73] analyzed the leaching toxicity of EMR following solidification treatment and combined construction waste materials with cement to manufacture burn-free bricks. The results demonstrate that, with an appropriate mixing ratio, unfired bricks that meet national standards (GB/T 30760-2024 [74]) can be produced. EMR-prepared unfired bricks have the advantage of large consumption, but market acceptance is low in some regions, and risks such as pollutant leaching must be strictly controlled.

4.1.3. Synergistic Preparation of Cementitious Materials

Wu et al. [75] utilized EMR, combined with steel residue and blast furnace residue, to develop a cementitious material activated by an alkaline activator (sodium hydroxide). Testing revealed that NH₄⁺ levels were nearly undetectable in the solution, and the concentration of heavy metal leaching was below the permissible standard. The maximum leaching concentration of Mn²⁺ was recorded at 0.089 mg/L, demonstrating a solidification efficiency of 99.99%. These results indicate that the cementitious material effectively encapsulates heavy metals, thereby mitigating ammonia nitrogen pollution in the environment.

Currently, the methods for recycling these resources have not been widely industrialized, primarily because researchers have not effectively pretreated the NH₃-N in EMR, and the solidification of heavy metals within the residue is not satisfactory. The introduction of alkaline substances in the brick-making process causes the NH⁴⁺-N in the EMR to decompose slowly after the bricks are formed, resulting in cracks and a reduction in the strength of the EMR brick. Furthermore, the inadequate solidification of heavy metals in the EMR hinders the leaching toxicity test. As a result, partial EMR bricks do not meet the requirements outlined in GB 34330-2017 [76], which restricts their application in brick-making.

Research on the resource utilization of EMR in the construction field still faces challenges, including the subpar performance of manganese residue products and limited scalability, which necessitate further investigation. Additionally, EMR contains a substantial amount of calcium sulfate dihydrate. While some studies have examined its application in cement retarders, research on other high-value-added gypsum products, such as calcium sulfate whiskers, remains limited. Calcium sulfate whiskers are cost-effective to produce and can significantly enhance the toughness, heat resistance, and wear resistance of polymer materials. Their diverse range of applications positions them as a green, environmentally friendly material with outstanding performance.

4.2. Recovery of Valuable Elements

The recovery of valuable metals from EMR primarily targets manganese, followed by lead, cadmium, arsenic, and other elements. This section just discusses the recovery of manganese. Most manganese exists in the form of soluble manganese ions, manganese carbonate, manganese dioxide, etc., accounting for about 3% of EMR. Given the substantial volume of residue, the potential recovery value is significant [77,78]. Manganese in EMR is recovered in the form of metallic manganese or manganese sulfate after treatment. Liu [79] proposed a method that involves grinding, strong magnetic roughing, and magnetic sweeping to obtain a concentrate containing 29.61% manganese from EMR, achieving a yield of 19.81% and a recovery rate of 60.81%. Liu et al. [80] utilized a clean water residue-washing and ammonium salt precipitation process to recover soluble manganese from EMR. The Mn²⁺ washing rate in the residue was 93.8%. By adding (NH₄)₂CO₃ and 0.4 mg/L of flocculant as precipitants, the manganese recovery rate reached 99.8%. Liu et al. [81] conducted countercurrent washing on the EMR filter cake. Initially, a low-concentration manganese-containing solution (11–14 g/L) was used to wash the filter cake in a diaphragm filter press, followed by an additional wash with 40 g/L of clean water. The results indicated a 1.05% reduction in the residual MnSO₄ content in the residue, with a recovery rate exceeding 56%. Metal recovery is suitable for residues with high manganese contents and has the advantage of high economic value, but it also has problems such as high acid consumption and heavy metals in wastewater that need to be dealt with. Manganese recovery is contingent upon the leaching technology. Consequently, future research on leaching should prioritize environmentally friendly and efficient leaching solutions and extractants.

4.3. Manganese Fertilizer

EMR contains essential nutrients, Mn, Se, K, Na, Fe, and B, which are vital for plant growth. These nutrients can enhance disease resistance, drought tolerance, and lodging resistance in crops, while also improving the overall yield [82,83]. Consequently, these beneficial properties can be utilized to develop agricultural fertilizers. During the preparation of manganese fertilizer, Liu [84] utilized oxalic acid as an additive to convert CaSO₄ in EMR into CaCO₃, thereby altering its chemical properties and preventing the hardening associated with CaSO₄. Zhu [85] proposed a composite fermentation method that employed EMR and biomass waste as raw materials, along with composite fermentation microorganisms, nitrogen-fixing microorganisms, and other organic microorganisms. This approach aimed to produce trace element fertilizers that can be fully absorbed by crops, thereby preventing the seedling and root burn caused by manganese-based fertilizers. Jiang et al. [86] introduced three additives to EMR and activated SiO₂ through high-temperature calcination and microwave digestion. The activated EMR met the standards for available Si and Mn²⁺/Mn⁴⁺ content, making it suitable as a silicon-manganese fertilizer for plants. Xu et al. [87,88] conducted a pot experiment applying manganese to radishes and wheat, finding that the plant height, chlorophyll content, and fruit weight increased in the treated crops compared to the control group, as shown in Figure 14 [89]. EMR fertilizer preparation is suitable for EMR with high nutrient elements and low toxic heavy metals. The applicable group is relatively small, and it has limitations. EMR must treat hazardous materials to eliminate toxicity before using them as a fertilizer for crops. To prevent plants from accumulating excessive heavy metals, which can lead to high heavy metal contents in food, it is important to consider the associated processing costs. However, these costs are relatively high, and the economic returns are often unsatisfactory.

4.4. Glass–Ceramic

Glass–ceramics are materials characterized by a uniform distribution of microcrystalline and glass phases, exhibiting exceptional properties, such as low density, impermeability, airtightness, high softening temperature, and excellent chemical and thermal stability, as well as mechanical strength and hardness. The main components of EMR are SiO₂, CaO, MgO, Al₂O₃, etc., and it is suitable as the basic raw material for microcrystalline glass. The main process flow is shown in Figure 15. After the steps of melting and water quenching, appropriate nucleating agents (such as TiO₂, ZrO₂, etc.) are added to control the heat-treatment process, promote the formation of crystal nuclei and crystal growth in the glass liquid, and finally form microcrystalline glass with specific properties. Qian [90] et al. proposed a patented method for producing brown EMR microcrystalline glass using EMR, CaCO₃, quartz sand, and MnCO₃ through sintering technology. The proportion of EMR can vary from 75% to 99.7% [91]. Liu et al. [92] conducted experiments to prepare microcrystalline glass using the sintering method. DSC/XRD and other performance tests demonstrated that the bending strength of manganese residue microcrystalline glass reached 106.82 MPa, exhibiting good acid and alkali resistance.

EMR-prepared glass–ceramics are simple, energy-saving, and environmentally friendly, with excellent product performance and high added value. Future research could concentrate on further optimizing the composition of EMR glass–ceramics to enhance their properties and value. However, EMR prepared glass–ceramics also face the problem of impurity removal, and the overall cost is relatively high. Furthermore, heat treatment leads to the decomposition of sulfate in EMR, releasing SO₂ and contributing to air pollution. This limitation hinders the advancement of EMR glass–ceramics.

4.5. Roadbed Materials

The roadbed, a crucial component of the pavement structure, consists of high-quality materials situated above the soil layer and a supporting layer below the surface, which is divided into upper and lower base layers. As high-consumption infrastructure materials, roadbed materials exhibit significant adaptability to various raw materials [93]. The preparation process for EMR roadbed materials is shown in Figure 16 [94]. Research indicates that incorporating 8% to 12% slaked lime into EMR results in a 7-day infinite lateral compressive strength, rendering it suitable for use as a roadbed material in highway construction. Additionally, the frost resistance, water resistance, and expansion coefficient of the manganese residue as roadbed backfill soil are comparable to those of clay, making it an effective substitute with enhanced performance. Consequently, utilizing manganese residue as a roadbed material decreases the consumption of natural clay while effectively repurposing the manganese residue. Zhang et al. [95] used red mud, carbide residue, and blast furnace residue to solidify and stabilize EMR. They subsequently added aggregate, cement, and water to prepare roadbed filler through stirring. The primary hydration products were CaAl₂SiO₈·4H₂O, CaAl₂O₄, Ca₂Al₂O₅, and Ca₃Al₂O₆. The Mn²⁺ leaching was measured at 0.00827 mg/L, with a solidification rate of 99.99%. The biggest advantage of EMR-prepared roadbed materials is it reduces the cost of raw materials and brings economic benefits. However, harmful substances may leach due to repeated vehicle rolling and rainwater infiltration. Therefore, the market acceptance is not high, and it causes the waste of valuable metal resources in EMR.

4.6. Adsorption Material

Rapid industrialization, population growth, and urbanization have intensified water pollution [96]. Advanced wastewater treatment has become a crucial strategy for mitigating water scarcity. EMR can enhance its porous structure by calcining or combining with other substances to form additional micropores. This structure exhibits a strong physical adsorption capacity for heavy metal ions and organic pollutants. Additionally, the alkaline components in EMR, such as CaO and MgO, dissolve in water to increase the pH, which facilitates the formation of hydroxide or carbonate precipitates (e.g., Cd(OH)₂, PbCO₃) for heavy metal ions. Furthermore, Mn³⁺ and Mn⁴⁺ can function as electron acceptors, reducing toxic valence states like As(III) and Cr(VI) to less harmful forms, or oxidizing organic pollutants such as phenols. Fu [97] synthesized an activated clay-particle adsorption material using an optimal material ratio of EMR:sludge:siliceous earth = 5:5:1. This material effectively adsorbs heavy metal ions in wastewater, particularly copper ions. The prepared clay particles met the requirements of GB 6566-2010 [98] for radioactive nuclides in Class A decoration and decoration materials (IRa ≤ 1.0, Ir ≤ 1.3). Sun et al. [99] developed an M-EMR adsorbent through a series of processes, including sequential immersion in an NaOH solution, ultrasonic processing, microwave-assisted aging, and vacuum filtration. Characterization revealed the presence of active sites such as Fe₃O₄, γ-FeOOH, and MnO₂ on M-EMR, which facilitate synergistic chemisorption and the oxidative conversion of arsenite and arsenate species. The preparation and adsorption process of EMR are illustrated in Figure 17a. Ma et al. [100] placed EMR in ultrapure water and homogenized it at 13,500 rpm for 15 min to obtain a hydrogel. The hydrogel was then solidified, oven-dried at 105 °C, ground to a particle size of less than 150 μm, and subjected to microwave heating at 800 °C for 2 h before cooling to room temperature, resulting in thermally activated EMR (T-EMR). T-EMR was utilized to adsorb and treat solutions containing Cd and Pb at pH = 6. The maximum adsorption capacities for Cd and Pb were found to be 35.97 mg/g and 119.88 mg/g, respectively. The adsorption mechanism involves three synergistic pathways: electrostatic interactions, cation-exchange processes, and surface precipitation reactions. The preparation and adsorption process of T-EMR is illustrated in Figure 17b. Wei [101] engineered Al₂O₃-modified EMR composites through high-temperature solid-state reactions with a 20 wt% Al₂O₃ loading, achieving an enhanced fluoride adsorption capacity. The modified material effectively removes F⁻ from water. While EMR demonstrates potential for wastewater remediation, intrinsic impurities may lead to the release of secondary pollutants during its application, thereby increasing remediation costs and operational complexity. Therefore, it is essential to remove harmful substances from EMR prior to its use.

5. Conclusions

The global accumulation of EMR presents a pressing challenge, primarily due to its complex composition of impurities and the low utilization of value-added products. In response, this paper critically reviews EMR harmlessness methods and comprehensive utilization approaches for EMR, encompassing chemical, physical chemistry, biological, and combined treatment technologies. To better integrate and utilize EMR, several suggestions and prospects are proposed:

(1) Among the existing purification technologies, electrochemical detoxification has garnered significant attention due to its high leaching efficiency, cost-effectiveness, and minimal industrial limitations. This technique exhibits exceptional potential for industrial applications and is considered the most promising method for large-scale implementation.

(2) EMR has been extensively studied for its potential applications in construction materials, particularly in cement production, brick manufacturing, wall material formulation, and roadbed construction. The incorporation of EMR into building materials is widely considered as the most effective utilization strategy. However, during the application process, the quantity of manganese residue used is relatively small. Enhancing the processing efficiency and increasing the utilization rate remain critical challenges that require further attention.

(3) The utilization and development of EMR must prioritize economic viability, operational safety, environmental sustainability, and scalability to meet contemporary industrial requirements. The intricate composition and hazardous constituents of EMR pose technical challenges in the development of large-scale processing technologies. Additionally, the complex processing procedures and the necessity for specialized equipment have impeded the effective recovery of resources. Determining how to implement feasibility experiments on a large scale in real-world situations is another issue that requires resolution.

(4) The electrolytic manganese industry has established essential economic foundations, but it has also created ongoing environmental challenges. EMR treatment processes often produce significant amounts of wastewater and exhaust gases, leading to secondary environmental contamination. The standards for non-hazardous resource-based products have not been uniformly established, which somewhat limits the utilization of these products.

At present, significant progress has been made in the purification of EMR, but substantial technical challenges remain. Future research should concentrate on developing efficient, environmentally friendly, and cost-effective purification technologies that have industrial applicability. Consequently, there is considerable scientific potential for the further exploration of EMR as a sustainable secondary resource through strategic utilization methods.